1. Field of the Invention
The present invention relates to an optical recording medium where a light beam is focused on the recording medium and information is reproduced by detecting light reflected from the recording medium. Particularly, the present invention relates to an optical recording medium having dual information surfaces.
2. Description of the Related Art
In recent years, optical recording media have become more and more important as a means for storing sound information data, image information data, and various information apparatus data because they can store and reproduce a large amount of data. There are still requirements for further increasing the capacity of the optical recording media and reducing the size of optical recording/reproducing apparatuses. In order to satisfy these requirements, the storage capacity of the optical recording media needs to be further increased.
Compact disks (CDs) having one information surface, for example, are known as a conventional read-only optical recording medium. The CD includes a spiral information track composed of convex and concave portions (pits) formed on a surface of a disk-shaped resin substrate with a thickness of 1.2 mm. A reflection film made of aluminum and the like and a protection film are formed on the resultant information surface of the substrate by sputtering and the like. An identification label is then printed on the protection film.
The storage capacity of such a CD is small because the CD has only one information surface. In order to increase the storage capacity, a recording medium where two disks are adhered together, such as a 5xe2x80x3 magneto-optical (MO) disk, has been commercialized. The 5xe2x80x3 MO disk is classified into two types; a disk having one information surface (one-sided disk) and a disk having two information surfaces (double-sided disk). The one-sided disk includes a spiral guide groove composed of convex and concave portions formed on a surface of a disk-shaped resin substrate with a thickness of 1.2 mm. A dielectric film, a magneto-optical recording material film, another dielectric film, and a reflection film made of aluminum and the like are formed in this order on the resultant information surface of the substrate by sputtering and the like. Another resin substrate with a thickness of 1.2 mm is then adhered to the reflection film. The double-sided disk includes a spiral guide groove composed of convex and concave portions formed on a surface of a disk-shaped resin substrate with a thickness of 1.2 mm. A dielectric film, a magneto-optical recording material film, another dielectric film, and a reflection film made of aluminum and the like are formed in this order on the resultant information surface of the substrate by sputtering and the like. The thus-fabricated disk is adhered with another disk fabricated in the same manner. Conventional 5xe2x80x3 MO disk recording/reproducing apparatuses are designed to receive both the one-sided disk and the double-sided disk to accomplish the recording and reproduction. The user can select the one-sided disk when information to be recorded is small or the double-sided disk when it is large. The 5xe2x80x3 MO disk apparatuses are generally provided with only one optical head. Accordingly, when the double-sided disk is used, the disk needs to be taken out and turned over to continue the recording or reproduction.
In general, the information density of a recording medium is determined by the pitch of an information track and the information density in the tracking direction, i.e., the information linear density. In order to increase the information density of the recording medium, the track pitch should be small, while the linear density should be large. In recent years, there have been studies to increase the density of the optical recording medium by reducing the thickness of the substrate to 0.6 mm, for example, to reduce the aberration of a light beam passing through the substrate due to a tilt of the disk.
However, the above conventional techniques have the following problems. In the case of the conventional double-sided optical recording medium, if both the top and bottom surfaces of the recording medium are illuminated with light beams so as to record information or reproduce recorded information, little space is left on the surfaces of the recording medium for printing an identification label. This is inconvenient for handling the recording medium. Also, when the conventional double-sided optical recording medium is used for an optical reproduction apparatus having only one optical head, the optical recording medium needs to be taken out from the apparatus and turned over to continue the reproduction. In order to continue the reproduction automatically, two optical heads disposed above and below the recording medium are required. An apparatus having two optical heads is large in size and its cost is high.
Another problem is that when a new optical recording medium thinner than the conventional optical recording media is commercialized to increase the density of the recording medium, such a new optical recording medium is not compatible with the conventional recording/reproduction apparatus.
The optical recording medium of this invention includes: a first substrate having a first information surface; a semitransparent reflection film formed on the first information surface of the first substrate; a second substrate having a second information surface; a reflection film formed on the second information surface of the second substrate; and an adhesive layer for adhering the first substrate and the second substrate so that the first information surface and the second information surface face each other, wherein the thickness of the first substrate is 0.56 mm or more, the thickness of the adhesive layer is 30 xcexcm or more, and the total thickness of the first substrate and the adhesive layer is 0.68 mm or less.
In one embodiment, the thickness of the first substrate is in the range of 0.56 mm to 0.6 mm, and the thickness of the adhesive layer is in the range of 40 xcexcm to 60 xcexcm.
In another embodiment, a recording material film is formed on the reflection film for the second substrate for recording and reproducing information.
In still another embodiment, the recording material film is made of a phase-change type recording material.
In still another embodiment, a label is formed on a surface of the second substrate.
In still another embodiment, a spiral track is formed on each of the first and second substrates, and the direction of the formation of the spiral track on the first substrate is the same as the direction of the formation of the spiral track on the second substrate when the spiral tracks are viewed from the side of a surface of the first substrate opposite to the first information surface.
In still another embodiment, a spiral track is formed on each of the first and second substrates, and the direction of the formation of the spiral track on the first substrate is reverse to the direction of the formation of the spiral track on the second substrate when the spiral tracks are viewed from the side of a surface of the first substrate opposite to the first information surface.
Alternatively, the optical recording medium of this invention includes: a first substrate having a first information surface; a semitransparent reflection film formed on the first information surface of the first substrate; a second substrate having a second information surface; a reflection film formed on the second information surface of the second substrate; and an adhesive layer for adhering the first substrate and the second substrate so that the first information surface of the first substrate faces a surface of the second substrate opposite to the second information surface, wherein the thickness of the first substrate is substantially the same as the thickness of the second substrate.
Alternatively, the optical recording medium of this invention includes: a first substrate having a first information surface; a semitransparent reflection film formed on the first information surface of the first substrate; a second substrate having a second information surface; a reflection film formed on the second information surface of the second substrate; an adhesive layer for adhering the first substrate and the second substrate so that the first information surface of the first substrate faces a surface of the second substrate opposite to the second information surface; and a label formed on the reflection film for the second substrate, wherein the thickness of the first substrate is substantially the same as the thickness of the second substrate.
Thus, the invention described herein makes possible the advantages of (1) providing an optical recording medium having dual information surfaces where a label can be easily printed on a surface of the recording medium, information can be automatically reproduced by use of one optical head, and the compatibility with an optical recording medium having one information surface can be secured, and (2) providing an optical recording medium which includes a substrate with a thickness different from the conventional standard but is compatible with conventional apparatuses so that information stored in the optical recording medium can be reproduced.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.